Functional component of
Cranial Nerves
EMBRYOLOGY
During early stages of development, the wall of the neural tube is made up of three layers:
a. The inner ependymal layer.
b. The middle mantle layer.
c. The outer marginal layer.
The mantle layer represents grey matter and the marginal layer, the white matter.
Functional components of cranial nerves:
• In the spinal cord, the somatic columns are the general somatic efferent and the general somatic afferent (sensory or posterior horn).
• These supply structures derived from somites.
The visceral columns are the general visceral efferent (motor) and the general visceral afferent (sensory).
These are autonomic columns and supply the viscera, vessels and glands.
In addition to the four functional columns differentiated in the spinal cord, there appear two more columns (a motor and a sensory) for the branchial apparatus of the head region, namely the special visceral (branchial) efferent and the special visceral afferent; and one column more for the special sense, namely the special somatic afferent. Thus a total of seven columns (3 motor and 4 sensory) are formed.
Each column, in its turn, breaks up into smaller fragments to form nuclei of the cranial nerves.
Transverse section of the hidbrain of an embro showing the arrangement of functional/nuclear columns of cranial nerve nuclei. (a) Spinal cord, and (b) in brain stem
NUCLEI
General Somatic Efferent (GSE) Nuclei
1. The oculomotor nucleus in the midbrain at the level of the superior colliculus.
2. The trochlear nucleus is situated in the midbrain at the level of the inferior colliculus.
3. The abducent nucleus is situated in the lower part of the pons.
4. The hypoglossal nucleus lies in the medulla.
Special Visceral Efferent/Branchial Efferent Nuclei These nuclei supply striated muscle derived from the branchial arches.
1. The motor nucleus of the trigeminal nerve lies in the upper part of the pons.
2. The nucleus of the facial nerve lies in the lower part of the pons.
3. The nucleus ambiguus lies in the medulla.
General Visceral Efferent Nuclei
1. The Edinger-Westphal nucleus lies in the midbrain in close relation to the oculomotor nucleus.
2. The lacrimatory nucleus lies near the salivatory nuclei.
3. The superior salivatory nucleus lies in the lower part of the pons.
4. The inferior salivatory nucleus lies in the lower part of the pons just below the superior nucleus.
5. The dorsal nucleus of the vagus is a long column extending into the open and closed parts of the medulla.
General Visceral Afferent Nucleus and Special Visceral Afferent Nucleus (Table 4.1)
The only nucleus in this category is the nucleus of solitary tract or tractus solitarius.
Its lower part receives general visceral sensations as follows:
a. Through the glossopharyngeal nerve from the tonsil, pharynx, posterior part of the tongue, carotid body and carotid sinus.
b. Through the vagus nerve from the pharynx, larynx, trachea, oesophagus and other thoracic and abdominal viscera.
Its upper part also receives sensations of taste (special visceral afferent) as follows:
a.From the anterior two-thirds of the tongue, and the palate except circumvallate papillae through the facial (VII) nerve in its superior part.
b.From the posterior one-third of the tongue through the glossopharyngeal nerve (IX) including the circumvallate papillae in its middle part.
c.From the posteriormost part of the tongue and from the epiglottis through the vagus (X) nerve in its inferior part.
General Somatic Afferent Nuclei
These are all related to the trigeminal nerve.
1.The main or superior sensory nucleus of the trigeminal nerve lies in the upper part of the pons.
2.The spinal nucleus of the trigeminal nerve descends from the main nucleus into the medulla. It reaches the upper two segments of the spinal cord.
3.The mesencephalic nucleus of the trigeminal nerve.
These nuclei receive the following fibres:
a. Exteroceptive sensations (touch, pain, temperature) from the skin of the face.
b. Proprioceptive sensations from muscles of mastication.
Special Somatic Afferent Nuclei
1. The cochlear nuclei (dorsal and ventral) that receive impulses of hearing through the cochlear nerve.
2. The vestibular nuclei (superior, spinal, medial and lateral) that receive fibres from the semicircular canals, the utricle and the saccule through the vestibular nerves.
Position of cranial nerve nuclear columns in brain stem
(b) Scheme to show the cranial nerve nuclei as projected on to the posterior surface of the brain stem with four vestibular nuclei, and (c) parts of nucleus of tractus solitarius: VII—
facial; ix glossopharyngeal and X—vagus
Contd.
